1. Field of the Invention
The present invention relates generally to combustion fuel vehicles, and, more particularly, to controlling fuel pressure in a common rail engine.
2. Description of the Related Art
Engines with common rail fuel injection systems are becoming increasingly popular. One important reason for this recognition is that varying the fuel pressure in the rail may reduce the average amount of emissions, such that, for each operating condition of the engine, an adequate fuel amount enters the engine. So far, the common-rail types of engines have mainly been developed for passenger cars. Now however, this technology is also introduced in heavy vehicles, such as trucks and busses, which are normally equipped with diesel engines. This places new requirements on the technical solutions.
DE 101 58 547 describes a fuel injection device for an internal combustion engine, wherein a reduced fuel pressure is enabled in a common rail by means of a piezo based actuator and a leak passage. An injected fuel pressure below the current pressure level in the common rail is here accomplished by discharging excessive fuel through the leak passage back to the fuel tank.
U.S. Pat. No. 6,024,064 discloses a high pressure fuel injection system for an internal combustion engine, wherein the fuel pressure in a common rail may be reduced electronically according to the engine operating conditions, for example in shifting-up of an automatic transmission.
Although fuel pressure may be reduced in connection with gearshift procedures taught in the prior art, under certain operating conditions, the fuel pressure level may still be too high when the original gear actually is released. One example of such an operating condition is when a vehicle drives uphill and a gearshift should be made to reduce the wheel torque. In order to enable a gear release, the engine torque must be lowered substantially, so that the torque in the gearbox practically attains a zero value. At least in diesel engines, the engine torque is approximately proportional to the amount of fuel injected into the engine's combustion chambers. Thus, if the fuel pressure is relatively high and a low torque is required (i.e. equivalent to a small amount of fuel), the opening time for the fuel actuator must be very short. Such short bursts of fuel often result in loud noises and undesired knockings, inter alia, because the overall opening time is insufficient to allow so-called “pilot injections.” Moreover, the interval during which the fuel actuator in the fuel injector feeds fuel into the combustion chamber is associated with certain tolerances, i.e., uncertainties as to the exact timing of the opening and the closure of the actuator. Hence, for short opening times these tolerances are comparatively large, perhaps in the same order as the opening time, and the resulting engine torque, therefore, becomes difficult to predict with a satisfying degree of certainty. In other words, a repeatable torque cannot be guaranteed at low levels of engine torque, for examples when releasing a gear in an automated manual gearbox, or anther type of automatically controlled gearbox. Consequently, sometimes a gearshift may be performed rather smoothly, whereas at other instances with similar conditions, uncomfortable slams and jerks may occur. Of course, this may annoy the driver and the vehicle's transmission system risk to deteriorate.